This invention relates to an air-fuel ratio control system for an internal combustion engine (hereinafter, referred to simply as "engine"), for controlling the air-fuel ratio of air-fuel mixture supplied to the engine.
FIG. 4 shows a fuel-air ratio control system for an engine, disclosed in Japanese Public Disclosure (Kokai) Nos. 59-221433 and 61-55336. Shown in FIG. 4 are an air cleaner 1, an air flow meter 2 for measuring intake air flow, a throttle valve 3, an intake manifold 4, a cylinder 5 of the engine, a coolant temperature sensor 6 for detecting the temperature of the cooling water, a crank angle sensor 7, an exhaust manifold 8, an exhaust gas sensor 9 for detecting the respective concentrations of the components of the exhaust gas, such as the oxygen concentration, a fuel injection valve 10, an ignition plug 11, a cylinder pressure sensor 13 for detecting the pressure in the combustion chamber of the engine and a control unit 15.
The crank angle sensor 7 generates reference angle pulses respectively at reference crank angles, namely, every 180.degree. rotation of the crankshaft for a four cylinder engine or every 120.degree. rotation of the crankshaft for a six-cylinder engine, and a unit angle pulse every unit angle rotation of the crankshaft, 1.degree.. The control unit 15 counts the unit angle pulses after the reception of the reference angle pulse to detect the crank angle at every moment. The engine speed can be detected through the measurement of the frequency or period of the unit angle pulses.
The crank angle sensor 7 of the fuel-air ratio control system shown in FIG. 4 is provided in a distributor. The control unit 15 comprises a microcomputer comprising a CPU, ROMs, RAMs, an I/O interface and the like. The control unit 15 processes the output signals of the air flow meter 2, the coolant temperature sensor 6, the crank angle sensor 7, the cylinder pressure sensor 13 and the like, and provides a fuel injection signal determined on the basis of the result of signal processing to control the fuel injection valve 10.
Shown, by way of example, in FIGS. 5(A) and 5(B) is the cylinder pressure sensor 13 comprising an annular piezoelectric crystal element 13A, an annular negative electrode 13B and a positive electrode 13C. FIG. 6 shows the position of the cylinder pressure censor 13 on the engine. The cylinder pressure sensor 13 is fastened to a cylinder head 14 with the ignition plug 11. The cylinder pressure sensor 13 generates an output signal proportional to the cylinder pressure.
This control unit 15 has a CPU which executes a control program as shown in FIG. 7 stored in a ROM at predetermined time intervals. Referring to FIG. 7, engine speed N and intake air flow Q are determined in step P1 from an output signal S3 of the crank angle sensor 7 and an output signal S1 of the air flow meter 2 respectively. In step P2, basic fuel injection quantity is calculated from engine speed N and intake air flow Q by using a formula: EQU T.sub.p =K(Q/N)
where T.sub.p is basic fuel injection quantity, K is a constant, Q is intake air flow and N is engine speed. In step P3, crank angle is determined from the output signal of the crank angle sensor 7. In step P4, a query is made to see if the determined crank angle corresponds to the bottom dead center (abbreviated to "BDC") of the crank of the cylinder in the suction stroke. Step P6 is executed when the response in step P4 is negative. When the response in step P4 is affirmative, step P5 is executed t store an output signal S6 of the cylinder pressure sensor 13 as cylinder pressure P.sub.t with the crank at the BDC in the suction stroke.
In step P6, a query is made to see if the crank angle corresponds to a predetermined crank angle after top dead center (abbreviated to "ATDC") in the compression stroke. The value of the predetermined crank angle is dependent on the ratio between the crank throw and the length of the crank connecting rod of the engine, and is, for example, 15.degree. in this example. When the response in step P6 is negative, the program returns to step P3 to repeat steps P3 through P6 until the response in step P6 becomes affirmative. When the response in step P6 is affirmative, the output signal S6 of the cylinder pressure sensor 13 is stored in step P7 as cylinder pressure P.sub.m at a crank angle 15.degree. ATDC.
Then, in step P8, the pressure ratio P.sub.m /P.sub.t is calculated and the calculated value of the pressure ration P.sub.m /P.sub.t is stored. In step P9, the pressure ratio P.sub.m P.sub.t is added to the cumulative sum .SIGMA.(P.sub.m /P.sub.t) of the pressure ratios calculated in the preceding control cycle to obtain the cumulative sum .SIGMA.(P.sub.m /P.sub.t) of a predetermined number of pressure ratios P.sub.m /P.sub.t. In step P10, the new cumulative sum .SIGMA.(P.sub.m /P.sub.t) and a cumulative sum .SIGMA.(P.sub.m /P.sub.t) used in the preceding fuel injection control cycle are compared, and an air-fuel ratio compensation factor .alpha. is calculated on the basis of the comparison. In step P11, a compensated fuel injection quantity T.sub.i is determined by using an expression: EQU T.sub.i =T.sub.p .times.(1+F.sub.t +KMR/100)+.alpha.+T.sub.s
where F.sub.t is a temperature compensation factor determined from the output signal S2 of the coolant temperature sensor 6, T.sub.s is a battery voltage compensation factor, and KMR is a high-load compensation factor obtained through table look-up using the engine speed N and the basic fuel injection quantity T.sub.p. The initial value of the air-fuel ratio compensation factor .alpha. is reset at "1"at the time of starting the engine.
Finally in step P12, the fuel injection valve 10 is operated by a signal S5 corresponding to the calculated, compensated fuel injection quantity T.sub.i.
Thus, according to the control program shown in FIG. 7, the air-fuel ratio is controlled in a feedback control mode by detecting the cylinder pressure P.sub.m at a crank angle at which the cylinder pressure is expected to reach a maximum, normalizing the cylinder pressure P.sub.m by the cylinder pressure P.sub.t at the BDC in the suction stroke, which is proportional to the load, an compensating the fuel injection quantity so that the value of the cumulative sum of a predetermined number of normalized values P.sub.m /P.sub.t reaches a maximum.
This air-fuel ratio control system, however, needs an expensive air flow meter and a still more expensive cylinder pressure sensor to measure intake air flow Q, which represents the load on the engine, and engine speed N to determine basic fuel injection quantity on the basis of the ratio Q/N. Furthermore, a comparatively long period of time is necessary for detecting cylinder pressure a predetermined times and summing the values of cylinder pressures delays the response of the air-fuel ratio control system during acceleration of the engine thereby causing deterioration in the performance of the engine.